1. Field of the Invention
The present invention relates to golf balls. Specifically, the present invention relates to golf balls including a solid core, a mid layer, and a cover.
2. Description of the Related Art
Golf players' foremost requirement for golf balls is flight performance. Golf players place importance on flight performance upon shots with a driver, a long iron, and a middle iron. An appropriate trajectory height is required in order to achieve a large flight distance. A trajectory height depends on a spin rate and a launch angle. In a golf ball that achieves a high trajectory by a high spin rate, a flight distance is insufficient. In a golf ball that achieves a high trajectory by a high launch angle, a large flight distance is obtained. Use of an outer-hard/inner-soft structure in a golf ball can achieve a low spin rate and a high launch angle.
JPH2-264674 (U.S. Pat. No. 5,072,944) discloses a golf ball that includes a core consisting of a center core and an outer layer. The center core is flexible, and the outer layer is hard. The core suppresses a spin rate.
JPH6-98949 (U.S. Pat. No. 5,516,110) discloses a golf ball having a constant hardness between: a point that is located at a distance of 5 mm from a central point; and a point that is located at a distance of 10 mm from the central point. A similar golf ball is also disclosed in JPH6-154357 (U.S. Pat. No. 5,403,010).
JPH7-112036 (U.S. Pat. No. 5,562,287) discloses a golf ball having a small difference between a central hardness and a surface hardness of a core. The core contributes to the resilience performance of the golf ball.
JPH11-253578 (U.S. Pat. No. 6,129,640) discloses a golf ball that includes a core, a mid layer having a specific gravity higher than the specific gravity of the core, and a cover having a hardness less than the hardness of the mid layer.
JP2002-764 (US 2002/0032077) discloses a golf ball having a great difference between a central hardness and a surface hardness of a core. A similar golf ball is also disclosed in JP2002-765 (US 2002/0019269).
JP2003-33447 (US 2003/0032501) discloses a golf ball that includes a core for which a rubber composition includes a polysulfide. The polysulfide contributes to the resilience performance of the golf ball.
JP2008-194473 (US 2008/0194357 and US 2008/0312008) discloses a golf ball having a great difference between a central hardness and a surface hardness of a core. A similar golf ball is also disclosed in JP2010-22504.
JP2009-297261 (US 2009/0312121) discloses a golf ball that includes a center, a mid layer having a hardness less than the surface hardness of the center, and a cover having a weight less than the weight of the mid layer.
For a cover, a highly rigid resin may be used. The highly rigid resin can suppress spin. In general, a highly rigid resin has inferior fluidity. Therefore, it is difficult to form this cover. It is particularly difficult to form a thin cover. For the purpose of improving moldability, a low-molecular-weight material may be blended with a highly rigid resin.
In general, a highly rigid resin has inferior impact resistance. For the purpose of improving impact resistance, a polymer including a rubber component may be blended with a highly rigid resin.
Golf balls in which a highly rigid resin is used are disclosed in JP2010-17414 (US 2010/0009776), JP2009-261791 (US 2009/0270203), and JP2009-261792.
In the golf ball disclosed in JPH2-264674, the structure of the core is complicated. The core produces an energy loss when being hit. In addition, the core has inferior durability.
In the golf ball disclosed in JPH6-98949, a range where the hardness is constant is narrow. The golf ball has inferior resilience performance. Similarly, the golf ball disclosed in JPH6-154357 also has inferior resilience performance.
In the golf ball disclosed in JPH7-112036, a spin rate is excessive. The golf ball has a small flight distance.
In the golf ball disclosed in JPH11-253578, the resilience performance is impaired by the mid layer. The golf ball has a small flight distance.
The golf ball disclosed in JP2002-764 has inferior resilience performance. Similarly, the golf ball disclosed in JP2002-765 also has inferior resilience performance.
In the golf ball disclosed in JP2003-33447, a spin rate is excessive. The golf ball has inferior flight performance.
In the golf ball disclosed in JP2008-194473, there is a zone in which a hardness decreases from the central point of the core toward the surface of the core. The golf ball has inferior resilience performance. In the golf ball, a spin rate is excessive. The golf ball has inferior flight performance. Similarly, the golf ball disclosed in JP2010-22504 also has inferior flight performance.
In the golf ball disclosed in JP2009-297261, the difference between the surface hardness and the central hardness of the core is not great. In the golf ball, a spin rate is excessive. The golf ball has inferior flight performance.
In a cover including a highly rigid resin and a low-molecular-weight material, the low-molecular-weight material bleeds. This bleeding impairs adhesion of a mark layer to the cover. This bleeding impairs adhesion of a paint layer to the cover. This bleeding further promotes separation of the cover from the mid layer.
In a composition in which a polymer including a rubber component is blended with a highly rigid resin, the polymer impairs fluidity. It is difficult to form a thin cover from this composition.
An object of the present invention is to provide a golf ball that has excellent flight performance and that can be manufactured easily.